The invention relates to a brake system as defined hereinafter.
Such brake systems, having a master brake cylinder via which brake circuits are supplied with brake fluid, in particular one brake circuit for the front wheels and one brake circuit for the rear wheels, are quite well known, as is a corresponding shutoff valve assembly for anti-skid control as shown in U.S. Pat. Nos. 5,013,097 and 5,039,176.
The brake design for vehicle axles must take into account the dynamic axle loads and their variations, in particular. In the ideal case, the brake force is always proportional to the deceleration by braking and the dynamic axle load. However, this "ideal" brake force distribution varies very markedly with the deceleration by braking.
According to the guidelines of the European Community and the European Common Market, the wheels of the rear axle must not lock before those of the front axle in deceleration by braking of the vehicle up to 0.8 G, because if the rear axle wheels lock first the vehicle will skid, while if the front axle wheels lock first, although the ability to steer the vehicle is lost, the vehicle remains relatively stable.
Ideal brake force distribution would mean that both axles contribute equally to the braking regardless of the load status, which is virtually unattainable. Typically, in order to meet the prescribed guidelines, the brake pressure for the rear axle is reduced or limited, or else a fixed vehicle design, with very small wheel brake cylinders on the rear axle, is selected. However, this results in overbraking of the front axle (not to be mistaken for locking, that is, skidding) compared with the rear axle. This is achieved at least up to deceleration by braking of 0.8 G. With more forceful deceleration by braking, however, a reversal of the order of locking may occur, because then the rear axle is overbraked. In their case, whichever axle is overbraked contributes disproportionately to the deceleration by braking.
Most deceleration by braking occurs in a range of under 0.3 G. Because of the constantly heavier load, the brake system wears faster on the front axle than on the rear axle. Furthermore, optimal decelerations by braking are not achieved; this is a problem particularly for heavy, high-powered vehicles, in which the load limit on the front axle limits the possible vehicle performance. When there is a functional anti-skid system that assures the stability of a vehicle, increasing underbraking of the rear axle compared with the front axle is not only unnecessary but in fact prevents optimal decelerations by braking, because the brake pressures remain markedly below the skid limit.